Digital delay circuit

ABSTRACT

A digital time delay circuit is provided in which fabrication process variations and temperature effects on the switching threshold level of digital circuits utilized in the timing delay circuits are substantially eliminated.

FIELD OF THE INVENTION

The invention pertains to delay circuits, in general, and to a digital delay circuit, in particular.

BACKGROUND OF THE INVENTION

One problem with prior art digital delay circuits is that the time delay produced by the circuit will vary in dependence on the threshold voltage of the logic circuit coupled to the time determining capacitor.

It is therefore desirable to provide a digital time delay circuit in which the logic threshold voltage does not influence the time delay.

SUMMARY OF THE INVENTION

In accordance with the principles of the invention, a digital time delay circuit is provided in which fabrication process variations and temperature effects on the switching threshold level of digital circuits utilized in the, timing circuits are substantially eliminated.

In accordance with the principles of the invention, a digital delay circuit for delaying a digital signal by a predetermined time delay, comprises first and second identical delay circuits. The first delay circuit has an input coupled to a first node. The second delay circuit has an output coupled to an output node. The first delay circuit is connected in cascade to the second delay circuit via an intermediate node.

The first delay circuit comprises a first capacitor, first and second switched current sources each coupled to the first capacitor and each having a control input coupled to the input node, the first switched current source sources current to the first capacitor when a signal at the input node is at a first level and said the switched current source sinks current from the first capacitor when the signal is at a second level. The source current and the sink current are each of the same magnitude. A first digital circuit has an input coupled to the capacitor and is coupled to an intermediate node. The first digital circuit has a first input threshold voltage level, and provides an output at the intermediate node that is at one level when the capacitor voltage is below the threshold voltage level and provides an output at the intermediate node that is at a second level when the capacitor voltage is above the threshold voltage level.

The second delay circuit comprises a second capacitor, third and fourth switched current sources each coupled to the second capacitor and each having a control input coupled to the intermediate node, the third switched current source sourcing current to the second capacitor when a signal at the intermediate node is at a first level and the second switched current source sinking current from the second capacitor when the signal at the intermediate node is at a second level, the sourcing current and the sinking current each being of the same magnitude; a second digital circuit having an input coupled to the second capacitor and having an output coupled to the output node, the second digital circuit having a second input threshold voltage level equal to the first input threshold voltage level, the digital circuit providing an output at the output node that is at one level when the second capacitor voltage is below the threshold voltage level and providing an output at the output node that is at a second level when the second capacitor voltage is above the input threshold voltage level; whereby the digital delay circuit provides a time delay that is independent of the threshold voltage level.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood from a reading of the following detailed description of the drawing in which like reference designators are used to identify like elements in the various drawing figures, and in which:

FIG. 1 is a representation of a prior art digital delay circuit;

FIG. 2 illustrates waveforms in the prior art digital delay circuit of FIG. 1;

FIG. 3 is a diagram of a digital delay circuit in accordance with the principles of the invention; and

FIG. 4 illustrates waveforms of the circuit of FIG. 3.

DETAILED DESCRIPTION

FIGS. 1 and 2 illustrate the problem to which the present invention is directed. Delay circuit 1 utilizes an electronic switch 101 that responds to an input signal to provide a time delay. Switch 101 controls the charge and discharge of a capacitor C from a current source 1. As can be seen from FIG. 2, when switch 101 opens, the voltage across capacitor C, voltage Vc, increases in accordance with the relationship of I/C=ΔV/Δt. As Vc increases from a first voltage level, zero volts, to a second voltage level, Vdd, it increases at a slope of I/C.

A digital circuit 103, represented by an inverter, is coupled to the capacitor C and is responsive to the capacitor voltage Vc. Digital circuit 103 has a threshold voltage Vth at which the output voltage of digital circuit 103 changes state.

Turning now to FIG. 2, the state of switch 101 is shown by curve 201, the voltage across capacitor C, Vc, is shown by curve 203, and the output voltage of digital circuit 103 is shown by curve 205.

When switch 101 is in a first or closed state, the output voltage Vo of digital circuit 103 is in a first or high state. When switch 101 changes state from its first or closed state to a second or open state, the voltage Vc begins to rise at a slope of I/C. The output voltage of digital circuit 103 initially remains in a first or high state. After a time delay Δt, voltage Vc reaches the switching threshold voltage Vth of the digital circuit 103 and the output voltage Vo of digital circuit 103 switches from its first or high state to its second or low output state.

The time delay Δt is dependent on the switching threshold voltage by the equation Δt=Vth (C/I). Unfortunately, the switching threshold voltage will vary with temperature and from device to device depending upon manufacturing process.

FIG. 3 shows an illustrative embodiment of a delay circuit 300 in accordance with the principles of the invention. Circuit 300 includes two identical delay stages 301, 303. Delay circuit 300 is provided on an integrated circuit 302 that may or may not include other circuitry.

Each delay stage 301, 303 includes input inverter 3011, 3021 and an output inverter 3013, 3023. Each inverter or digital circuit 3011, 3021, 3013, 3023 has the same input switching threshold voltage Vth and the input switching threshold voltages of the inverters or digital circuits 3011, 3021, 3013, 3023 all vary identically with temperature.

Input inverters 3011, 3021 are coupled to the control input of corresponding electronic switches 3015, 3025. Electronic switches 3015, 3025 function as single pole double throw type switches and are used to selectively couple either a current source 3017, 3027 or a current sink 3019, 3029 to respective capacitors C1, C2. Current sources 3017, 3027 and current sinks 3015, 3025 each source or sink the same level of current 1.

Turning now to FIG. 4, waveforms are shown for the input voltage Vin, capacitor voltage V_(A), capacitor voltage V_(B), voltage V₁, and output voltage Vout. The waveforms for capacitor voltage V_(A) and capacitor voltage V_(B) are superimposed on each other.

The time delay, Δtrise, between the rising edges of Vin and Vout as well as the time delay, Δtfall, between the falling edges of Vin and Vout are a function of the current I, the capacitance C of capacitors C1, C2 and the supply voltage Vdd which are all independent of the threshold voltage of the inverters 3011, 3021, 3013, 3023. More specifically Δtrise=Δtfall=Vdd(C/I). In addition, Δtrise is the sum of the time tar for the voltage V_(A) to rise from zero to Vth plus the time tbf for the voltage V_(B) to fall from Vdd to Vth; and Δtfall is the sum of the time taf for the voltage V_(A) to fall from Vdd to Vth plus the time tbr for the voltage V_(B) to rise from zero to Vth.

As the voltage Vth varies with temperature or because of process parameters, the effect of a variable Vth is eliminated by utilizing both rise and fall times from zero to Vth and from Vdd to Vth to determine the time delay.

The digital delay circuit 300 delays a digital signal by a predetermined time delay independent of the threshold voltage of the digital inverters or circuits utilized. Delay circuit 300 comprises an input node 3200; an output node 3300; a first delay circuit 301 coupled to the input node 3200; a second delay circuit 303 coupled in cascade to the first delay circuit 301 and coupled to the output node 3300, The first delay circuit 301 comprises a first capacitor C1, first and second switched current sources 3017, 3019 each coupled to the first capacitor C1 and each having a control input coupled to the input node 3200. The first switched current source 3017 sourcing current to the first capacitor C1 when a signal at the input node 3200 is at a first level 0 volts. The second switched current source 3019 sinking current from the first capacitor C1 when the signal at node 3200 is at a second level. The sourcing current and the sinking current each being of the same magnitude 1. The first digital circuit 3013 has an input coupled to capacitor C1 and an output at an intermediate node 3400. First digital circuit 3013 has a first input threshold voltage level Vth. First digital circuit 3013 provides an output at intermediate node 3400 that is at one level, zero, when the capacitor voltage V_(A) is below the threshold voltage level Vth and provides an output at intermediate node 3400 that is at a second level Vdd when the capacitor has a voltage above the threshold voltage level. A second delay circuit 303 comprises a second capacitor C2, third and fourth switched current sources 3027, 3029 each coupled to the second capacitor C2 and each having a control input coupled to the intermediate node 3400. Third switched current source 3027 sources current to the second capacitor C2 when a signal at the intermediate node 3400 is at a first level and the fourth switched current source 3029 sinking current from the second capacitor C2 when the signal at the intermediate node 3400 is at a second level. The sourcing current and the sinking current are each of the same magnitude 1.

A second digital circuit 3023 has an input coupled to the second capacitor C2 and has an output coupled to the output node 3300. Second digital circuit 3023 has an input threshold voltage level equal to the first input threshold voltage level Vth. Second digital circuit 3023 provides an output at the output node 3300 that is at one level when the second capacitor voltage V_(B) is below the threshold voltage level Vth and provides an output at the output node 3300 that is at a second level when the second capacitor voltage V_(B) is above the input threshold voltage level Vth. Digital delay circuit 300 provides a time delay that is independent of the threshold voltage level Vth of the digital circuits or inverters 3011, 3021, 3013, 3023.

The invention has been described in conjunction with a specific illustrative embodiment. It will be understood by those skilled in the art that various changes, substitutions and modifications may be made without departing from the spirit or scope of the invention. It is intended that all such changes, substitutions and modifications be included in the scope of the invention. It is not intended that the invention be limited to the illustrative embodiment shown and described herein. It is intended that the invention be limited only by the claims appended hereto, giving the claims the broadest possible scope and coverage permitted under the law. 

1. A digital delay circuit for delaying a digital signal by a predetermined time delay, comprising: an input node; an output node; a first delay circuit coupled to said input node; a second delay circuit coupled in cascade to said first delay circuit and coupled to said output node; said first delay circuit comprising a first capacitor, first and second switched current sources each coupled to said first capacitor and each having a control input coupled to said input node, said first switched current source sourcing current to said first capacitor when a signal at said input node is at a first level and said second switched current source sinking current from said first capacitor when said signal is at a second level, said sourcing current and said sinking current each being of the same magnitude; a first digital circuit having an input coupled to said capacitor and having an intermediate node, said first digital circuit having a first input threshold voltage level, said first digital circuit providing an output at said intermediate node that is at one level when said capacitor has a voltage below said threshold voltage level and providing an output at said intermediate node that is at a second level when said capacitor has a voltage above said threshold voltage level; and said second delay circuit comprising a second capacitor, third and fourth switched current sources each coupled to said second capacitor and each having a control input coupled to said intermediate node, said third switched current source sourcing current to said second capacitor when a signal at said intermediate node is at a first level and said second switched current source sinking current from said second capacitor when said signal at said intermediate node is at a second level, said sourcing current and said sinking current each being of the same magnitude; a second digital circuit having an input coupled to said second capacitor and having an output coupled to said output node, said second digital circuit having a second input threshold voltage level equal to said first input threshold voltage level, said digital circuit providing an output at said output node that is at one level when said second capacitor has a voltage below said threshold voltage level and providing an output at said output node that is at a second level when said second capacitor has a voltage above said input threshold voltage level; whereby said digital delay circuit provides a time delay that is independent of variations of said threshold voltage level.
 2. A digital delay circuit in accordance with claim 1, comprising: an integrated circuit having said first and second digital circuits formed thereon.
 3. A digital delay circuit in accordance with claim 2, wherein: said input threshold voltage level is temperature dependent.
 4. A digital delay circuit in accordance with claim 2, wherein: said input threshold voltage level is fabrication process dependent
 5. A digital delay circuit in accordance with claim 4, wherein: said input threshold voltage level is temperature dependent.
 6. A digital delay circuit in accordance with claim 1, wherein: said first and said second digital circuits each comprise an inverter.
 7. A method for providing time delays in a digital circuit, said time delays being independent of temperature and fabrication process effects on digital circuit threshold levels, said method comprising: providing a two stage delay circuit responsive to an input signal that switches between a first voltage level and a second voltage level, operating a first stage of said two stage delay circuit to provide an intermediate delayed signal delayed from said input signal by a first portion of a predetermined time delay; determining said first portion of a predetermined time delay from the time for a timing voltage to change state from one of said first or second voltage levels to a threshold voltage level; operating a second stage of said two stage delay circuit to provide an output signal delayed from said intermediate delayed signal such that said output signal is delayed from said input signal by said predetermined time delay; utilizing said second stage to delay said intermediate delayed signal by a second portion of said predetermined time delay; determining said second portion from the time for a timing voltage to change state from the other of said first or second voltage levels to said threshold voltage level; whereby the summation of said first portion and said second portion is said predetermined time delay.
 8. A method in accordance with claim 7, comprising: fabricating said first and second stages as part of a single integrated circuit.
 9. A method in accordance with claim 8, comprising: providing in each of said first and said second stages a digital circuit having an output switchable between said first and second voltage levels at said threshold voltage level. 